The invention relates generally to electrode films. The invention relates particularly to electrode films for optoelectronic devices.
Organic optoelectronic devices include organic light emitting devices, organic photovoltaic devices, organic electrochromic devices, and the like. Organic optoelectronic devices operate by injection of charges, which recombine to result in radiation of energy as in a light emitting device, or by separation of charges as in a photovoltaic device. Many organic optoelectronic devices, such as organic light emitting devices (OLEDs), may require transparent electrodes. For example, when OLEDs serve as display or lighting devices, it is necessary for the generated light to leave the device. Therefore, at least one of the two electrodes is fabricated out of, transparent conducting material, such as tin doped indium oxide (ITO). Thin ITO films have been widely used as transparent electrodes due to their high transparency, desirably 80% or greater transmittance in the wavelength region from about 400 nm to about 700 nm and low bulk resistivity, desirably 1×10−3 ohm-cm or lower.
Conventionally, organic optoelectronic devices have been built on glass substrates. For glass substrates, electrode films such as ITO films, with low resistivity, have been easily achieved at substrate temperatures (Ts) higher than 200° C., using a variety of deposition techniques such as sputtering, electron-beam evaporation, and pulsed laser deposition. The use of plastic substrates for organic optoelectronic devices will help reduce the weight, the fragility of these devices, and make it possible to fabricate flexible devices. Plastics, particularly transparent plastics, normally have a glass transition temperature (Tg) below 150° C. Also, the coefficient of thermal expansion (CTE) of plastics (typically 60˜100 ppm/° C.) is much higher than that of ITO (<20 ppm/° C.). A high temperature process may disadvantageously result in huge residual stress at the interface between the electrode material and the plastic substrate and cause the ITO layer to crack. Therefore, for plastic substrates, it may be advantageous to deposit the electrode material such as ITO, at a lower Ts due to the heat-sensitivity of plastics. However, by implementing conventional deposition techniques performed at low Ts, it is difficult to achieve ITO films with low resistivity and high transparency.
Accordingly, a technique is needed to address one or more of the foregoing problems in optoelectronic devices, such as organic optoelectronic devices.